1. Field of the Invention
The present invention relates to a transparent molding composition comprising a polyamide blend and being suitable for production of transparent, printable items.
2. Description of the Related Art
The current standard process for decoration of external areas on automobiles is painting. However, this procedure firstly generates high manufacturing costs, resulting from provision of specific plant and the operating cost associated therewith for the automobile producer, and secondly causes pollution of the environment. Pollution of the environment derives by way of example from solvent constituents released from the paints used, and also from accumulation of paint residues, which have to follow correct disposal routes.
Another factor is that painting has only limited suitability for decorating the surfaces of plastics components, which in recent years have become more popular in automobile construction, because of the saving in weight and cost.
The process of painting plastics components which are components of bodywork can, for example, be carried out on-line, the plastics part being subjected to a paint treatment identical with that for the metallic components. This leads to a uniform color, but is attended by high temperatures resulting from the cathodic electrodeposition method conventional here, and this makes the selection of material more difficult. In addition, identical adhesion of the paint formulation has to be ensured on very different substrates. If the process of painting the plastics parts is carried out in a separate step (known as off-line painting), comprising process conditions more advantageous for plastics, the problem of colormatching arises, meaning that the shade achieved on the metal has to be matched precisely. However, the differences in substrate and in the underlying paint formulation that can be used, and process conditions, make this very difficult to achieve. If there is a color difference prescribed via the design, a serious disadvantage that remains is provision of a second set of painting equipment for the plastics parts and the cost associated therewith, and additional time required for manufacture of the automobile also has to be considered. Direct use of the untreated, generally injection-molded plastics parts is aesthetically disadvantageous, because surface defects resulting from the process, such as weld lines, air inclusions, and also necessary reinforcing fillers, such as glass fibers, are clearly discernible here. This is intolerable in visible regions. Consequently, improvement of surface quality has to be undertaken, for example in the context of a painting process, frequently requiring much work for pretreatment via polishing and application of relatively thick layers of a primer.
One proposed solution consists in the use of multilayer plastics films, used to cover the components and then requiring no painting. The bond between substrate and decorating film here can be achieved via a number of manufacturing processes. By way of example, the film can be laminated to the substrate, or it is possible to select a process of in-mold coating by an injection-molding process, in which the film is placed in the injection mold during component production. The concept of a film as carrier of decoration is also in line with a trend toward individualization of design elements on automobiles. Specifically, this trend leads to a wider range of models in the manufacturing process, but with a reduction in the number of respective components manufactured per series. The use of films permits rapid, problem-free design change, and can therefore meet this challenge. An important factor here is that the film complies with the standards demanded in the automobile industry with respect to surface properties (class A surface), solvent resistance, and appearance. Films with these properties likewise have good capability for use in the design of interior surfaces in automobiles.
Decorative films of this type are in principle known. EP 0 949 120 A1 describes by way of example decorative films with a transparent outer layer comprising polyurethane, polyacrylate, fluoropolymer, or mixtures comprising fluoropolymer and polyacrylate. WO 94/03337 and EP 0 285 071 A2 disclose similar decorative films.
The utility model DE 295 19 867 U1 describes a decorable film comprising a copolyamide, which comprises the following monomer units: laurolactam, and also caprolactam, and/or hexamethylenediamine/dicarboxylic acid. Although these copolyamides are generally transparent and are also easy to decorate, problems constantly occur during extrusion to produce moldings or films from copolyamides of this type. In particular, deposits form on the injection mold or extrusion die or on the take-off rolls, and the necessary cleaning work often interrupts production. Furthermore, films of this type have inadequate heat resistance, and there is therefore a risk of deformation during decoration by means of sublimation print or thermal diffusion print. The temperature at which decoration has to be carried out is therefore lower than would actually be desirable in these processes. An excessive proportion of short-chain comonomers moreover leads to undesirably high water absorption of the films produced therefrom, causing unacceptable warpage of the finished parts on exposure to moisture. One application sector for decorative films is as carriers of decoration for example for topcoats of sports equipment, such as skis or snowboards, or of household items. Here, single-layer films are often used, printed on the top- or underside.
In the article by M. Beyer and J. Lohmar, Kunststoffe 90 (2000) 1, pp 98-101, examples of printable films comprising PA12 molding compositions are given, but films of this kind have disadvantages with regard to surface gloss and inadequate heat resistance.